Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to a method for reducing silicon oxide and for increasing the solubility of oxygen in gold, silver and tin.
Some of the metals which are desirable for doping silicon for achieving particular electrical properties are not ideal metals for practicing thermal gradient zone melting processing of silicon wafers. Such metals are gold, silver and tin. These three metals do not wet silicon in air or in an argon atmosphere.
The inability of gold, silver and tin to wet silicon in an air or an argon atmosphere leads to the spontaneous breakup of the molten wire and grid structures comprising these materials during initiation of melting the deposited metal to form the melt to be migrated during the penetration stage of thermal migration.
It appears that the reason why gold, silver and tin do not wet silicon is because of the presence of a thin layer of silicon oxide on the silicon surface that was generated during wafer processing in air. Since neither tin, gold nor silver can reduce silicon oxide, the silicon oxide layer persists under the deposited layers of these metals. Because these metals will not wet the silicon oxide, they tend to ball up on the wafer surface and thus cause breakup of the wire, or line, and grid structure.
Gold, silver and tin are important metals for imparting lifetime control and N-type conductivity to silicon wafers.